Project description:A whole transcriptome study was performed on Sulfolobus islandicus REY15A actively undergoing CRISPR spacer acquisition from the crenarchaeal monocaudavirus STSV2 in rich (TYS) and basal (SCV) media over a 6 day period. Spacer acquisition preceded strong host growth retardation, and changes in viral transcript abundance and virus copy numbers showed significant differences between the two media. Results showed that rich medium favoured CRISPR-Cas immunity generation.
Project description:Key to CRISPR-Cas adaptive immunity is maintaining an ongoing record of invading nucleic acids that are encountered, a process carried out by the Cas1-Cas2 complex that integrates short segments of foreign genetic material (spacers) into the CRISPR locus. It is hypothesized that Cas1 evolved from casposases, a novel class of transposases. We show here that casposase integration in vitro recapitulates several properties of CRISPR-Cas integrases. The X-ray structure of Methanosarcina mazei casposase bound to DNA representing the product of integration reveals a tetramer with target DNA bound snugly between two dimers in which single-stranded casposon end binding resembles that of spacer 3'-overhangs. The differences between transposase and CRISPR-Cas integrase are largely architectural, and it appears that evolutionary change involved changes in protein-protein interactions to favor Cas2 binding over tetramerization and the separation of Cas1 dimers. This, in turn, led to preferred integration of single spacers over two transposon ends.